If current trends hold, we will end up with an eleven day heat wave by the time next Sunday gets here. It looks pretty likely at this point that the region is dealing with the summer's longest duration of heat...with today serving as the halfway point of that streak. 90+ temperatures look likely through Sunday, with Wednesday featuring an increase of humidity while Thursday may bring a further increase in heat while humidity drops back a notch. Despite the yin or yang in temperature and humidity, the prospect of heat index values at or slightly above 100 around the Delaware Valley on Wednesday and Thursday afternoon results in Excessive Heat Watches getting issued by the National Weather Service.
It wouldn't surprise me if those watches turn to warnings later this afternoon, with a watch getting hoisted for Friday...with a rinse and repeat in the watch and warning cycle expected tomorrow as well.
The eleven day heat wave (90 or higher) would tie for the 7th longest streak on record in Philadelphia (there have been nine streaks of eleven or more days). The last time we had a ten day streak was in July-August 2002. The last eleven day or longer streak was in July-August 1999. It's been a while since we've been down this path...our Heat Wave Index projections place this heat wave (should it go through to eleven days) as the 8th or 9th worst on record...we had a ten day heat wave in 1973 that had a bit more "bite" to it. It also would not be the worst 11 day heat wave on record -- the July 1993 heat wave that killed 118 was the worst 11 day episode on record according to our scale.
Will we see temperatures hotter than what we experienced last Friday (98 degree high)? If we do, the best chance would be on Saturday (the core of heat overhead, limited chance of thunderstorms). That might be our shot to go for 100 if we're going to get there in this pattern.
The pattern is showing signs, however, of breaking down next weekend. A cold front should push down from Canada late Saturday and into Sunday, bringing thunderstorms to the region on Sunday as it passes through before stalling to our south. Sunday's temperature forecast is the toughest of the streak in terms of potential 90 degree days...we might not hit 90 if the front gets in early enough or if it stalls overhead and keeps clouds in place. Right now, our call is for 90 but things *may* change on that.
After the front pushes through early next week, temperatures will dip a bit and more seasonable weather will ensue next week. Believe it or not, there might not be any 90's through mid month after this weekend. While it still be summery and temperatures will be in the 80's on a daily basis, we could see a nice respite from the heat around here. Keep your fingers crossed on that!